Security laminates are traditionally used to protect documents or packages to ensure that the underlying items are not altered. Security laminates are particularly useful on identification cards such as driver's licenses and passports, and on other important documents such as certificates of title. Security laminates are also useful as protective labels on medications, video cassettes, and compact discs. Four features are particularly important when producing and using security laminates. First, once applied to an article it is important that the laminate is difficult to remove to ensure that the underlying item is not altered or subjected to tampering. Second, a desirable laminate is difficult if not impossible to duplicate by counterfeiters. Third, if tampering occurs it is important to quickly and accurately recognize an altered or counterfeit laminate. Fourth, it is important that manufacturing and application costs of the laminates are not prohibitively expensive.
Security laminates are constructed of various materials. In order to overcome the problem of counterfeit documents, manufacturers have made it exceedingly difficult for counterfeiters to duplicate the security laminate used on a particular document. Some constructions require special viewing devices to discern whether or not the laminate was subjected to tampering. Such laminates are disclosed in U.S. Pat. Nos. 4,630,891, 5,080,463, and 5,169,707. These laminates were multilayer films comprising a monolayer of microspheres at least partially embedded in a binder layer, typically with associated specular or diffuse reflective layers, to impart a retroreflective appearance to the document. The microsphere-containing layer is printed with a customer-specified logo. The retroreflective layer is then adhesively bound to the document. These constructions are difficult to imitate by counterfeiters because it is difficult to imitate the customer-specific retroreflective printing. However, in order to determine authenticity of the laminates, special retroreflective viewing equipment is required to view the customer-specific printing. Tampering is indicated by damage to the continuity of the retroreflective layer, visible as dark lines or discoloration which does not retroreflect. Therefore, it is difficult to detect authenticity of the document with the naked eye. This type of laminate is also expensive to manufacture due to multiple layers and a sophisticated process control which is necessary to manufacture the laminate.
Multilayer film constructions containing intermediate layers having optical properties such as holograms or kinegrams are often used in security laminates. EP-A-0 000 401 describes such laminate constructions. If the security laminate in these constructions is disturbed due to tampering, the holograms are destroyed. Tampering is therefore readily apparent when viewing the document with the naked eye. However, the film layer containing the holographic optical pattern is not transparent and is also quite expensive. These laminate constructions do not overcome all of the problems associated with security laminates because it is undesirable to cover an entire document or package with an opaque, expensive multilayer film. Further, it is necessary to properly register the opaque hologram on the document so as not to obscure any underlying data.
One relatively inexpensive laminate construction is described in DE-C-25 11 367. This reference discloses laminating a document between two films and further including a pattern of adhesion-reducing coating either on the film or on the document before laminating. Once the layers are laminated, little or no bond exists in those places coated with the adhesion-reducing coating. Attempts to tamper a document laminated with such a construction ordinarily result in destruction of the article to which the laminate is adhered. However, skilled tamperers are capable of removing the film without damaging the underlying article by using heat.
U.S. Pat. Nos. 4,876,123 and 5,082,702 disclose tamper-proof multilayer films which are not transparent. During attempts to tamper, the multilayer constructions are destroyed and both sides of the separated film layers display an originally concealed colored print. Tampering with these constructions is apparent to the unaided eye and the films are also impossible to laminate together without visible damage. However, laminates having these constructions are not useful on identification documents because they are not transparent.
U.S. Pat. No. 5,153,042 discloses a tamper-indicating labelstock or security laminate comprised of a transparent facestock, a release coating attached to one surface of the facestock for providing an indicia, a polymer coated on the facestock and release coating, a frangible metal layer and an adhesive layer. The labelstock is easily broken when tampering occurs which reveals the indicia printed by the release coating. Construction of this laminate requires a flood coating of primer over the release coating. Since this laminate is intended for use as a labelstock, it is easily delaminated. Such a construction is not ideal for security laminates used to protect documents because the laminate is not durable enough and would split prematurely under everyday abuse. Also, this type of construction is easily delaminated with heat rendering tampering possible.
A transparent self-adhesive film which includes printing is described in DE-A-28 03 434. When the film is lifted during tampering, the printing remains on the document. If a photograph is exchanged for the original, the tamperer must only reproduce the printing on the new photograph which is a relatively easy task for a skilled tamperer. Constructions such as those described do not overcome the problem of imitation by tamperers.
Security laminates have also included an intermediate layer of ink-containing microcapsules as described in FR-A-2 451 081. The capsules break during tampering which results in discoloration of the document. Although tampering is readily apparent to the unaided eye, the microcapsules create at least two disadvantages which are unacceptable in the field of transparent security laminates. The microcapsules reduce the transparency of the film and the microcapsules break during normal use of the document. Thus, such a laminate is too fragile for use on identification cards subjected to daily abuses which would cause the microcapsules to fracture.
Another method of manufacturing tamper-proof documents is described in DE-C-29 52 322. A document is coated with a polyethylene film coated with an adhesive which requires cross-linking by ultraviolet irradiation. The resultant laminate provides a high degree of security, however, it is again prohibitively expensive to place irradiation equipment and the corresponding protection equipment at every document distribution location.
None of the references described above solve the problems with security laminates. That is, a security laminate which is inexpensive to manufacture and to apply to documents and has sufficient durability for identification cards. Further, the security laminate must be transparent, difficult to imitate by counterfeiters and must readily evidence tampering, even when heat tampering is employed.